The present invention pertains to drilling structures and particularly to a drilling structure for use in Arctic waters, particularly the Beaufort Sea. As is well known, the Beaufort Sea is relatively shallow and subject to annual movement of sheet ice. Breaking up of the annual formation of ice in the Beaufort Sea occurs in June and July and depending upon the winds this broken ice will drift with the tides. The ice is strong in compression but relatively weak in tension. Thus, it is suggested that offshore structures used in the Beaufort Sea have sloping side walls which will cause the ice to ride up and fail in tension, breaking into smaller pieces which would not present a hazard to the structure.
Also, structures used in the Beaufort Sea must be firmly anchored in position so that they can resist the forces created by movement of the ice. In the past, it has been suggested that the platforms be relatively heavy and ballasted into position so that they would be firmly anchored on the bottom. Thus anchored, they were capable of resisting the moving ice and the tendency of the moving ice to tip the platform over. In this respect, it has been suggested that the platforms have a frustoconical shape so that they have a relatively large surface area at the bottom to resist the tipping action of the ice movement while still presenting an inclined surface to assist in breaking up the sheet ice. While this is a partial solution to the problem, it does result in relatively heavy platforms which are expensive to build and difficult to move. Further, due to the weight of the platform which increases its draft, its use is limited to rather deep waters.
In an attempt to assist in anchoring offshore arctic platforms, U.S. Pat. No. 4,037,424 suggests that reversible acting thrusters be positioned within chambers formed in the hull and communicating with the bottom of the floatable hull to draw soil from the subsoil into recesses in the hull and anchor the hull more firmly in place. The patent also suggests that the thrusters could be reversed when it is desired to release the hull from the ocean floor and move it to a new location. While this is a partial solution to the problem, the suggested hull is still a heavy structure and the main anchoring force is obtained from the weight of the hull.
A further problem occurs in offshore arctic structures when it is necessary to move the structure to prevent its damage from ice floes at certain times of the year. Under these conditions, it is necessary to remove the structure from the ocean floor and tow it to a safe location. This, of course, necessitates abandoning the well to possible damage from the ice floe. Since the possibility of damage to the well exists some means must be provided for protecting the wellhead structure which normally projects above the surface of the sea floor.